1. Field of Invention
The present invention relates to microelectromechanical system (MEMS) device. More particularly, the present invention relates to the MEMS microphone with sensitivity trimming circuit.
2. Description of Related Art
MEMS microphone has a sensing diaphragm to sense the vibration of sound in pressure. The sensing diaphragm forms as a part of a sensing capacitor, so that the sound is converted into electric signal.
FIG. 1 is drawing, schematically illustrating the sensing mechanism by a diaphragm. In FIG. 1, a diaphragm 100 shown in perspective view, such as a thin film. In side view, the diaphragm 100 has a radius R and is at a free state 102, so that the diaphragm 100 is in a flat plane. When an air pressure, produced by a sound signal, is applied to the diaphragm 100, the diaphragm 100 becomes a bending state. Causing a displacement of the diaphragm 100. As can be understood, the diaphragm 100 vibrates with the pressure variance of the sound. If the diaphragm 100, as one electrode, is formed with a fixed electrode as a capacitor, the displacement of the diaphragm 100 change the capacitance followed the capacitance relation of C=∈ A/d, in which d is changed with the source pressure. As a result, the sensing capacitor can sense the sound.
FIG. 2 is a circuit, schematically illustrating a circuit of the MEMS microphone. In FIG. 2, the diaphragm capacitor 122 of the MEMS microphone 120 has two electrodes with one electrode in a diaphragm. A capacitor biasing voltage source 128 connecting to the diaphragm capacitor 122 through a high impendence resistor 129 provides constant charges Q on the diaphragm. The diaphragm senses the sound pressure and produces a displacement Δd, causing change of capacitance. The known laws are Q=CV and C=∈ A/d. In which ∈ is Permittivity of free space, A is capacitor area. Under the condition of fix charge Q, then a relation can be obtained as ΔV/VPP=Δd/d, in which d is the distance of the two electrode of the capacitor at the natural state without sound. Δd is the displacement due to pressure change on the diaphragm. The voltage signal ΔV is amplified by the source follower circuit 124 with amplification Av1 and the amplifier 126 with amplification Av2 to obtain the Vout=Av1*Av2*ΔV.
However, the mechanical property of sensing diaphragm of the diaphragm capacitor 122 manufactured by standard CMOS process is variable due to the variance of thin film thickness and residual stress, causing the sensitivity is not easy to be fabricated to have the preset level. This cause the performance MEMS microphone is not stable.
How to obtain a stable sensitivity is one of issues, needed to be considered. The solution may be taken by developing the precise fabrication process, causing high cost.